This is apparently a common scam; my wife's car was used by a body shop which was supposed to be repairing it. Given all the potential liability issues, as well as the violation of the trust which was placed in the shop, I would be concerned if it were my car.
Taking a customer's car home is not a suitable test drive, in my opinion. I agree with Gina - idling is a fast route to high engine temperatures in many cases, and it doesn't require taking the vehicle off the shop's lot.
I expect that most of us would be upset if an RV repair shop sent our trailer on a camping trip with a staff member "to test it" when they were supposed to be repairing it, and that's about the same situation.
1979 Boler B1700RGH, pulled by 2004 Toyota Sienna LE 2WD
Information is good. Lack of information is not so good, but misinformation is much worse. Check facts, and apply common sense liberally.
STATUS: No longer active in forum.